Senmyo Calendar (a variation of the lunar calendar that was created in ancient China) (宣明暦)
Senmyo Calendar is a kind of Chinese calendar. It is a lunar-solar calendar used in China and Japan. It is officially called the Chokei Senmyo Calendar.
The Japanese calendar is based on the old lunar calendar. The Christian Era is based on the Julian calendar until October 4, 1582, and it has been based on the Gregorian calendar since October 15, 1582.
The calendar was used for 71 years from 822 to 892 in Tang China.
Senmyo Calendar replaced the Taien reki (Taien Calendar) and the Goki reki (Wuji calendar) on February 3, 862, and was used for 823 years until February 3, 1685.
The Senmyo Calendar was replaced by the Jokyo reki (Jokyo calendar) on February 4, 1685.
The Senmyo Calendar, compiled by Joko in Tang China, was a superior calendar at that time. The calendar was particularly better at forecasting solar and lunar eclipses.
Bokkaishi introduced the Senmyo Calendar in 859. It replaced the Taien reki and the Goki reki. It is said that the calendar was strongly recommended by OKASUGA no Manomaro, who was a reki hakase (master of reki - calendar).
After that, the calendar was not replaced by a new calendar because of the weakened imperial court and the sluggish study of the calendar. Because the calendar was used for a long period of time, errors had been accumulated.
The calendar used to be compiled solely by the imperial court, and books concerning calendar calculations were secret except for the Onmyoryo (Bureau of Divination). However, because the Senmyo Calendar was used for quite a long period, the calendar began to be distributed among the general public, and also published. In addition, because the imperial court started to lose power after the Kamakura period, and it became difficult to distribute calendars produced in Kyoto to regional areas, calendars (private calendars) based on a method of making calendars of the Senmyo Calendar began to be produced independently in various parts of the country.
However, because the Kadenokoji family, which was in charge of calendar making in the imperial court during the Sengoku period (the period of warring states), was cut off, the compilation of the calendar in Kyoto became disorganized. As a result, a confusion occurred between the calendar compiled in Kyoto and private calendars such that the calendars used different Chijun-ho (a calendar technique for setting the leap month and leap day). For instance, a leap month scheduled for January 1583 according to Kyo-goyomi (a calendar made by Onyoryo - Bureau of Divination - in Kyoto) occurred in December 1582 in the estates of the Gohojo, the Uesugi, and Satomi clans where a private calendar Mishima-goyomi (a calendar made by the Kawai family and distributed from the Mishima-taisha Shrine) was adopted. Especially in Shinano Province where the Sanada and the Ashita clans adopted Mishima-goyomi, and the Suwa and the Ogasawara clans adopted Kyo-goyomi (a calendar made by the Onyoryo - Bureau of Divination - in Kyoto), confusion occurred because a leap month occurred differently within the same province.
In addition, 24 divisions of the old calendar and the first day of the month began to be indicated two days earlier than the actual dates in the early part of the Edo period (it can also be said that the calendar compiled in the ninth century had only an error of two days). Because of this, the Jokyo reki (Jokyo calendar) was compiled by Harumi SHIBUKAWA as a new calendar to replace the Senmyo Calendar.